Field of the Invention and Related Art Statement
The present invention relates to an ultrasonic diagnostic apparatus for diagnosing characteristics of a specimen such as a living body by using an ultrasonic wave, and a pulse compression apparatus for use therein.
Heretofore, there has been proposed in, for instance Japanese Patent Application Publication Kokai Sho No. 58-73,345, an ultrasonic diagnostic apparatus using the pulse compression method which has been utilized in the pulse radar. In this known ultrasonic diagnostic apparatus, there are merits that the detectable range can be made long and the distance resolution can be increased, but has the following demerits mainly due to the fact that a special SAW (Surface Acoustic Wave) filter is used as a matching filter for effecting the pulse compression.
That is to say, since the ultrasonic diagnostic apparatus uses the low frequency and a wide frequency range, it is rather difficult to design the SAW filter satisfying such requirements. Further, in the ultrasonic diagnostic apparatus, in case of inspecting the living body having a large damping effect with the aid of the pulse reflection method, the filter characteristics have to be modified in the dynamic mode in accordance with the range from which the reflected echo is returned. However, in the SAW filter the filter characteristics are fixed by the pitch and length of the electrodes, so that it is practically impossible to obtain the SAW filter having the dynamic filter characteristics.
In order to remove the above problem, the applicant has proposed, in Japanese Patent Application Publication No. 63-233,369 (corresponding to U.S. patent application No. 107,497 now U.S. Pat. No. 4,788,981), the pulse compression apparatus for use in the ultrasonic diagnostic apparatus in which the echo signal is compressed by deriving the correlation with the aid of a correlation circuit by effecting the convolution-integrating in the time domain between the echo signal waveform and an impulse response waveform (reference wave). In this pulse compression apparatus for the ultrasonic diagnosis, since the SAW filter is not used and thus the pulse compression is carried out by deriving the correlation between the echo signal and the reference wave, the pulse compression is effected without being affected by the frequency and range, and thus the dynamic filter characteristics can be realized. Therefore, it is possible to realize the ultrasonic diagnostic apparatus having the high resolution and sensitivity.
The inventor has conducted various experiments and has found that the pulse compression apparatus disclosed in the above mentioned Japanese Patent Application Publication No. 63-233,369 has the following drawback.
In this ultrasonic diagnostic apparatus, reference wave data for the pulse compression has been set on the basis of an impulse signal actually received by the ultrasonic vibrating element and passed through a matching filter and has been stored in a memory circuit. The reference wave signal is derived by reading the reference wave data out of the memory circuit. Therefore, the previously set and stored reference signal data becomes not optimum due to the secular variation of the ultrasonic vibrating element and various circuit elements. This results in the decrease in the quality of ultrasonic image. In other words, in the above mentioned ultrasonic diagnostic apparatus, it is rather difficult to adjust or change the reference wave signal.
In case of effecting the ultrasonic diagnosis, it is usual to prepare a plurality of ultrasonic vibrating elements having different frequency characteristics and any desired ultrasonic vibrating element is selectively used. In this case, there have to be prepared a plurality of reference wave signals each corresponding to respective elements, and these reference wave signals are stored in the memory circuit. In such a case, it is further cumbersome to preset or change a plurality of reference wave signals.